Cancer is the second leading cause of death in the United States, exceeded only by heart disease. Drugs that are used to treat cancer tend to be toxic at their therapeutic dose levels, commonly causing severe and even life-threatening adverse effects. Current anticancer drugs must also be administered intravenously. Consequently, nearly all cancer chemotherapy must be administered in a hospital or clinic. An additional problem with most current cancer chemotherapy is that cancers frequently develop resistance to the drugs, so that recurrence of disease is common.
For patients who have been diagnosed with cancer, cytotoxic chemotherapy is considered an essential part of the management of the disease, but resistance to chemotherapeutic drugs is unfortunately a common development in cancer. Although the mechanisms of resistance to chemotherapy are not fully understood, the cellular mechanisms thus far implicated in the development of drug resistance are the same as those that protect normal tissues from toxicity. Furthermore, the efficacy of cytotoxic chemotherapeutics is ultimately limited by their narrow therapeutic index. Therefore, it is unlikely that any breakthrough in the treatment of cancer will come about as a result of a cytotoxic approach. There is accordingly an urgent need for new noncytotoxic therapies that are safer and more effective than those currently available, and that, furthermore, will improve both survival rate and the quality of life for cancer survivors.
Recurrence is a potential threat for anyone who is diagnosed and treated with cancer, and up to 50% of patients with recurrent cancer will eventually have metastatic disease, which is often fatal. Therefore, for patients who have been treated for early stage cancer, once stabilization of the disease has been achieved, consideration must be given to adjuvant chemopreventive therapy to suppress disease for as long as possible. Since chemopreventive therapeutics are used on a long-term basis, there is a serious need for agents with three key characteristics: good tolerability, oral bioavailability, and long-term safety.
Furthermore, primary prevention is the optimal way to address any disease, and this is particularly true of cancer. Continuing advances in identification and validation of intermediate biomarkers, along with risk factors (e.g., genetic susceptibility or life-style) and exposure biomarkers, offer opportunities to more accurately assess the risk that any given individual may develop cancer. The aforementioned advances also enable a more precise identification of patient groups at an elevated risk for development of cancer, e.g., patients who may be in an otherwise undiagnosed phase of a carcinogenic process that could ultimately be fatal. Accordingly, the development of an effective cancer preventive (“chemopreventive”) agent for high risk individuals is of utmost importance. Since chemopreventive agents may be given to relatively healthy subjects for extended time periods, the long-term safety of such drugs is essential.
Currently, none of the available methods for treating cancer, such as breast cancer, ovarian cancer and prostate cancer, meet all of these important criteria.
Breast cancer is one of the most prevalent types of cancer. Although breast cancer research has developed at a rapid pace over the last decade, breast cancer remains a common and devastating disease and the second leading cause of cancer-related deaths in women in the United States. Many breast tumors appear to follow a predictable clinical pattern, initially being responsive to endocrine therapy and cytotoxic chemotherapy but ultimately exhibiting a phenotype resistant to both modalities. Although the mechanisms responsible for hormone resistance of tumors remain unclear, experiments revealed that when a tumor composed of mixed populations of cells with different sensitivities to hormones was deprived of hormones, the autonomous cell types could keep growing, and inevitably the tumor growth progressed from hormone sensitive to hormone independent. Since cellular heterogeneity of estrogen receptor (ER) distribution is seen in most cases of ER-positive breast cancer, the promising treatment strategy and drugs should achieve maximal growth inhibition of both estrogen-dependent and estrogen-independent breast tumor cells at the same time.
New therapeutic agents are also needed for the treatment of ovarian cancer. Ovarian cancer has a high mortality-to-incidence ratio, is usually asymptomatic until it is diagnosed in advanced stages, and quickly develops resistance to existing chemotherapeutics. The advent of paclitaxel (Taxol) as a component of first-line and salvage therapies has further improved response rates and prolonged survival, but resistance to chemotherapeutic drugs is a common development in ovarian cancer. These chemoresistant tumor cells frequently develop a broad cross-resistance to multiple drugs, and virtually all patients in whom multiple drug resistance has developed do not survive.
With the advent of prostate-specific antigen (PSA) testing and increased public awareness, approximately 75% of prostate cancer patients now present with clinically localized disease at the time of initial diagnosis. Although detection of organ-confined disease provides the most realistic opportunity for cure, the curative potential of all presently accepted local therapies (i.e., surgery and radiation therapy) remains disappointing, while treatment-associated side effects have been shown to seriously impair sexual, urinary, and bowel function for most patients. As diagnostic modalities and screening advance, continued increases in the incidence of prostate cancer and the shift to an earlier patient age and tumor stage at diagnosis are expected in the years to come. Clearly, there is an urgent need to identify and implement novel therapeutic agents to improve cancer control while minimizing associated morbidity.
It is, therefore, of utmost importance to develop new anticancer agents that are not only effective in treating a range of cancers, but also exhibit low toxicity and have a wide therapeutic window, such that an agent allow long-term treatment to maximize disease control. An ideal anticancer agent would also be easily administrable outside of a clinical setting; orally active compounds would be particularly attractive in this regard. Ideal agents would also be useful prophylactically in patients at risk of developing cancer, or at risk of cancer recurrence, in addition to their utility in therapeutic methods.
One route to discovering safe anticancer agents is to search for dietary compounds that have anticancer properties, then to modify them to enhance their anticancer effects while retaining their safe biological profile. Known dietary compounds with anticancer activity include certain indoles, particularly indole-3-carbinol (I3C), that are found abundantly in cruciferous vegetables such as broccoli, cabbage, cauliflower, and Brussels sprouts. I3C is highly acid sensitive and it can be converted by gastric acid to form several metabolites in stomach. The four I3C metabolites shown below—3,3′-diindolylmethane (3,3′-DIM), indolo[3,2-b]carbazole (ICZ), 2-(indol-3-ylmethyl)-3,3′-diindolyl-methane (LT), and 5,6,11,12,17,1 8-hexahydro-cyclonona [1,2-b:4,5-b′:7,8′-triindole (CT)—have been identified as having antitumor activity:

A number of in vitro and in vivo studies have shown I3C and its metabolites to have significant activity in preventing and treating estrogen-related cancers, including cancers of the breast (Bradlow et al. (1991) Carcinogenesis 12:1571-1574) 1991), cervix (Jin et al. (1999) Cancer Res. 59:3991-3997; Bell et al. (2000) Gynecol. Oncol. 78:123-129), and endometrium (Kojima et al. (1994) Cancer Res. 54:1446-1449). One mechanism for this activity appears to be the antiestrogenic properties of I3C and its metabolites. These properties include P450 cytochrome-mediated induction of 2-hydroxylation of estradiol, resulting in the production of non-estrogenic metabolites (Michnovicz et al. (1990) J. Natl. Cancer Inst. 82:947-949; Michnovicz et al. (1997) J Natl. Cancer Inst. 89:718-723). Additionally, I3C appears to directly suppress estrogen-induced signaling by the estrogen receptor in breast cancer cells (Meng et al. (2000) J Nutrition 130:2927-2931).
It is known that I3C and its metabolites also possess anticancer activities that are independent of their antiestrogenic properties. For example, these compounds have been found to suppress the migration and invasion of breast cancer cells by mechanisms that include up-regulation of the BRCA1 gene, E-cadherin (a regulator of cell-cell adhesion) and PTEN (a tumor suppressor gene) (Meng et al. (2000) J. Mol. Med. 78:155-165). I3C and its metabolites have also been found to inhibit cell cycle progression at G1 by inhibiting cyclin-dependent kinase (Cover et al. (1998) J. Biol. Chem. 273: 3838-3847) in both estrogen receptor-positive (ER+) and estrogen receptor-negative (ER−) breast cancer cells. They have also been shown to induce apoptosis in breast cancer cells by means independent of the p53 gene (Ge et al. (1999) Anticancer Res. 19:3199-3203), and down-regulation of the apoptosis inhibitory protein Bcl-2 (Hong et al. (2002) Biochem. Pharmacol. 63: 1085-1097). In addition, I3C and its metabolites appear to be protective against colon cancer by stimulating apoptosis in precancerous cells and by preventing potentially cancer-causing intracellular DNA damage (Bonnesen et al. (2001) Cancer Res. 61:6120-6130). I3C has also been demonstrated as useful in suppressing growth and inducing apoptosis in prostate cancer cells by mechanisms independent of its antiestrogenic properties (Chinni (2001) Oncogene 20:2927-2936). I3C has additionally been found to exhibit efficacy against cancers of the liver (Tanaka et al. (1990) Carcinogenesis 11: 1403-1406) and lung (Morse et al. (1990) Cancer Res. 54:1446-1449). One study has found that topically administered I3C was effective in suppressing chemically induced carcinogenesis in mouse skin (Srivastava et al. (1998) Cancer Lett. 134:91-95). Furthermore, a clinical trial found that orally administered I3C was effective in treating the proliferative but non-cancerous disease respiratory papillomatosis (Rosen et al. (1998) Otolaryngol. Head Neck Surg. 118:810-815).
Results obtained by Jin et al. (1999), supra, and Chen et al. (2001) J Nutr. 131(12):3294-3902, also establish I3C as having antiviral activity, insofar as I3C was found to be effective in treatment of cervical and cervical-vaginal cancers associated with human papillomavirus (HPV). Chen et al. also indicate that I3C has been found to have clinical benefits for laryngeal papillomatosis.
The present invention is the result of extensive, systematic research in the design of novel indoles in the form of structural analogs of the primary I3C metabolites that have optimized to enhance their anticancer activity and retain their safe biological profile. To the best of applicants' knowledge, the compounds and methods of the invention are completely unknown and completely unsuggested by the art.